The metal oxide transparent conductive material, like indium tin oxide (ITO), nickel oxide (NiO) has been widely used on opto-electronic devices, such as thin film transistor liquid crystal display (TFT-LCD), organic light emitting device (OLED) and light emitting device especially in InGaN series material. It is used as a conductive window layer of the opto-electronic device for current spreading and light transmission. The major technical concerns is to seek low and stable operating forward voltage (Vf), since it is not easy to form ohmic contact between the indium tin oxide and p type GaN.
The forward voltage is lowered by applying an agent layer between the indium tin oxide and p type GaN. For example, the U.S. Pat. No. 5,977,566 filed by Okazaki et al. uses some metal, such as Mg, Ni, Au, Zn or Ti, as an agent layer. Besides, the U.S. Pat. No. 6,078,064 filed by Ming-Jiunn et al. uses a high doping p type contact layer, such as InGaN, GaAs, AlGaAs or GaP, as the agent layer.
However, the agent layer between the indium tin oxide and the p type GaN contact layer will absorb output light intensity, and the forward voltage is unstable during operation because the high doping concentration inside the agent layer results in carrier diffusion between the agent layer and the contact layer.
In addition, using an indium tin oxide layer as a current spreading layer to cover the Ni/Au transparent conductive layer so as to enhance the light output have been a well-known technique in the production of semiconductor opto-electronic devices. For example, the U.S. Pat. No. 5,925,897 filed by Oberman et al. adds a thin Au/Ni layer between the indium tin oxide layer and the p type InGaN contact layer. The U.S. Pat. No. 6,465,808 filed by Lin et al. uses a doted transparent conductive layer to decrease the absorption area, so as to achieve the objective of increasing the light output. Besides, the U.S. Pat. No. 6,287,947 filed by Ludowise et al. adds a multi-layered transparent conductive layer between the indium tin oxide layer and the p type GaN contact layer.
However, the aforementioned inventions all suffer a problem of poor reproducibility of forward voltage and current because of the difference of surface roughness of epiwafer or the variation of hydrogen passivation effect.